1. Field of the Invention
The present invention relates to a field emission backlight unit, and more particularly, to a field emission backlight unit having improved luminous efficiency.
2. Description of the Related Art
In general, flat panel displays can be classified into light emitting displays and light receiving displays. Light emitting type displays include Cathode Ray Tubes (CRTs), Plasma Display Panels (PDPs), and Field Emission Displays (FEDs). Light receiving displays include Liquid Crystal Displays (LCDs). LCDs are light-weight and have low power consumption. However, the LCDs are light receiving displays that are not self-luminescent but form an image using incident light from the outside, and thus cannot provide an image in dark places. To solve this problem, backlight units are installed on a rear side of the LCD.
In conventional backlight units, a Cold Cathode Fluorescent Lamp (CCFL) has been used as a linear light source, and a Light Emitting Diode (LED) has been used as a point light source. However, the backlight units generally have a complicated structure, high manufacturing costs, and high power consumption due to reflection and transmission of light. In particular, as the size of the LCD increases, the uniformity of brightness cannot be easily obtained.
As such, in order to solve the problem, field emission backlight units having a flat light emission structure have been exploited. The field emission backlight units have a lower power consumption than conventional backlight units using CCFLs and provide comparatively uniform brightness even in a wider emission region. The field emission backlight units can also be used for illumination.
In a field emission backlight unit, an upper substrate and a lower substrate are separated from each other and face each other. An anode is formed on a bottom surface of the upper substrate, and a phosphor layer is formed on a bottom surface of the anode. A plurality of cathodes and a plurality of gate electrodes which are arranged in parallel to one another are formed on a top surface of the lower substrate. The cathodes and the gate electrodes are alternately formed on the same plane. The cathodes and the gate electrodes are formed of a thin film having a thickness of about 1000-3000 Å. A plurality of emitters formed of an electron emission material, for example, carbon nanotubes (CNTs), are disposed at both edges of the cathodes. A plurality of spacers for maintaining a uniform spacing between the upper substrate and the lower substrate are disposed therebetween. In the above structure, as voltages are supplied between the cathodes and the gate electrodes, electrons are emitted from the emitters disposed on the cathodes, and the emitted electrons are accelerated by the voltage supplied to the anode and excite the phosphor layer so that visible light is emitted.
However, in such a field emission backlight unit, since the cathodes and the gate electrodes are formed of a thin film on the same plane, an electric field formed around the emitters formed on the cathodes is greatly affected by the voltages supplied to the anode as well as the voltages supplied to the gate electrodes. Thus, in order to maximize the luminous efficiency of the phosphor layer, if a high voltage is supplied to the anode, the electric field formed around the emitters is affected by the voltage supplied to the anode so that excessive electrons are emitted from the emitters. As such, the current that flows through the anode increases. This results in degradation of the luminous efficiency of the backlight unit.